There are various uses for silver sulfate, including as a synthetic reagent; a source of silver in the preparation of catalysts, plastic composite materials and various platinum complexes; as well as a source of silver in some photographic processes. Recently silver sulfate has been incorporated into plastics and facial creams as an antimicrobial and antifungal agent. To satisfy the demands of many modem applications, reduction of particle sizes of materials to the micron and nanometer size ranges is often required to take advantage of the higher surface area, surface energy, reactivity, dispersability and uniformity of these size particles; as well as the uniformity and smoothness of coatings made thereof, optical clarity due to reduced light scatter, etc., inherent in these forms of matter. In addition, with the miniaturization of the physical size of many objects and devices, a similar limitation on the physical size of material components is now commonly encountered.
Silver sulfate is a commercially available material that is produced by conventional aqueous precipitation methods. The reaction of equimolar amounts of aqueous solutions of silver nitrate and sulfuric acid to from silver sulfate was described by Th. W. Richards and G. Jones, Z. anorg. Allg. Chem. 55, 72 (1907). A similar precipitation process using sodium sulfate as the source of sulfate ion was reported by O. Honigschmid and R. Sachtleben, Z. anorg. Allg. Chem. 195, 207 (1931). An alternate method employing the immersion of silver metal in a sulfuric acid solution was also reported by O. Honigschmid and R. Sachtleben (loc. cit.). Precipitation of finely divided silver sulfate from an aqueous solution via the addition of alcohol was later reported by H. Hahn and E. Gilbert, Z. anorg. Allg. Chem. 258, 91 (1949). Silver salts are widely known to be thermally and photolytically unstable, discoloring to form brown, gray or black products. Silver ion may be reduced to its metallic state, or oxidized to silver oxide, or react with sulfur to form silver sulfide. Silver sulfate has been observed to decompose by light to a violet color.
The antimicrobial properties of silver have been known for several thousand years. The general pharmacological properties of silver are summarized in “Heavy Metals”—by Stewart C. Harvey and “Antiseptics and Disinfectants: Fungicides; Ectoparasiticides”—by Stewart Harvey in The Pharmacological Basis of Therapeutics, Fifth Edition, by Louis S. Goodman and Alfred Gilman (editors), published by MacMillan Publishing Company, NY, 1975. It is now understood that the affinity of silver ion to biologically important moieties such as sulfhydryl, amino, imidazole, carboxyl and phosphate groups are primarily responsible for its antimicrobial activity.
The attachment of silver ions to one of these reactive groups on a protein results in the precipitation and denaturation of the protein. The extent of the reaction is related to the concentration of silver ions. The diffusion of silver ion into mammalian tissues is self-regulated by its intrinsic preference for binding to proteins through the various biologically important moieties on the proteins, as well as precipitation by the chloride ions in the environment. Thus, the very affinity of silver ion to a large number of biologically important chemical moieties (an affinity which is responsible for its action as a germicidal/biocidal/ viricidal/fungicidal/bacteriocidal agent) is also responsible for limiting its systemic action—silver is not easily absorbed by the body. This is a primary reason for the tremendous interest in the use of silver containing species as an antimicrobial, i.e., an agent capable of destroying or inhibiting the growth of microorganisms, such as bacteria, yeast, fungi and algae, as well as viruses. In addition to the affinity of silver ions to biologically relevant species that leads to the denaturation and precipitation of proteins, some silver compounds, those having low ionization or dissolution ability, also function effectively as antiseptics. Distilled water in contact with metallic silver becomes antibacterial even though the dissolved concentration of silver ions is less than 100 ppb. There are numerous mechanistic pathways by which this oligodynamic effect is manifested, i.e., ways in which silver ion interferes with the basic metabolic activities of bacteria at the cellular level to provide a bactericidal and/or bacteriostatic effect.
A detailed review of the oligodynamic effect of silver can be found in “Oligodynamic Metals” by I. B. Romans in Disinfection, Sterilization and Preservation, C. A. Lawrence and S. S. Bloek (editors), published by Lea and Fibiger (1968) and “The Oligodynamic Effect of Silver” by A. Goetz, R. L. Tracy and F. S. Harris, Jr. in Silver in Industry, Lawrence Addicks (editor), published by Reinhold Publishing Corporation, 1940. These reviews describe results that demonstrate that silver is effective as an antimicrobial agent towards a wide range of bacteria, and that silver can impact a cell through multiple biochemical pathways, making it difficult for a cell to develop resistance to silver. However, it is also known that the efficacy of silver as an antimicrobial agent depends critically on the chemical and physical identity of the silver source. The silver source can be silver in the form of metal particles of varying sizes, silver as a sparingly soluble material such as silver chloride, silver as a moderately soluble salt such as silver sulfate, silver as a highly soluble salt such as silver nitrate, etc. The efficiency of the silver also depends on i) the molecular identity of the active species—whether it is Ag+ ion or a complex species such as (AgSO4)−, etc., and ii) the mechanism by which the active silver species interacts with the organism, which depends on the type of organism. Mechanisms can include, for example, adsorption to the cell wall which causes tearing; plasmolysis where the silver species penetrates the plasma membrane and binds to it; adsorption followed by the coagulation of the protoplasm; or precipitation of the protoplasmic albumin of the bacterial cell. The antibacterial efficacy of silver is determined, among other factors, by the nature and concentration of the active species, the type of bacteria; the surface area of the bacteria that is available for interaction with the active species, the bacterial concentration, the concentration and/or the surface area of species that could consume the active species and lower its activity, and the mechanisms of deactivation.
One proposed use of silver based antimicrobials is for textiles. Various methods are known in the art to render antimicrobial properties to a target fiber. The approach of embedding inorganic antimicrobial agents, such as zeolites, into low melting components of a conjugated fiber is described in U.S. Pat. Nos. 4,525,410, and 5,064,599. In another approach, the antimicrobial agent can be delivered during the process of making a synthetic fiber such as those described in U.S. Pat. Nos. 5,180,402, 5,880,044, and 5,888,526, or via a melt extrusion process as described in U.S. Pat. Nos. 6,479,144 and 6,585,843. In still yet another process, an antimicrobial metal ion can be ion exchanged with an ion exchange fiber as described in U.S. Pat. No. 5,496,860.
High-pressure laminates containing antimicrobial inorganic metal compounds are disclosed in U.S. Pat. No. 6,248,342. Deposition of antimicrobial metals or metal-containing compounds onto a resin film or target fiber has also been described in U.S. Pat. Nos. 6,274,519 and 6,436,420.
In particular, the prior art has disclosed formulations that are useful for highly soluble silver salts having aqueous solubility products, herein referred to as pKsp, of less than 1. In general, these silver salts require the use of hydrophobic addenda to provide the desired combination of antimicrobial behavior and durability. Conversely, it is also known that very insoluble metallic silver particles, having a pKsp greater than 15, would require hydrophilic addenda to provide the desired combination of antimicrobial behavior and durability. There exists a need to provide sparingly soluble silver salts in the range of pKsp from about 3-8, which can be highly efficient in antimicrobial and antiviral behavior when incorporated directly into plastics and polymeric materials.
The use of an organo-sulfate or organo-sulfonate additive as a means of controlling the particle size of precipitated silver sulfate is disclosed in U.S. Pat. No. 7,261,867. However, the inclusion of substantial amounts of organic character in additives to silver sulfate materials has been shown to compromise the thermal stability. For example, when compounded into melt-processed polymers such as polypropylene, an unacceptable degree of discoloration of the composites often results.
Parasiticidal preparations of metal alkylsulphates and metal detergent sulphonates are described by Duperray in GB 1,082,653. Disclosures include the preparation of silver salts of alkylsulphates (specifically, silver laurylsulfphate and silver lauroylamino-ethylsulphate) and the silver salt of an alkylarylsulphonate (specifically, silver dodecylbenzenesulphonate). These compounds are prepared by reacting an excess of the alkylsulphate or alkylarylsulphonate with silver hydroxide, to form a neutral salt (or 1:1 adduct). While considerable efficacy in destroying parasitic protozoa such as coccidiae and histomones resulted when these compounds were added to the drinking water of diseased chickens, turkeys and cattle, neutral silver salts of these kinds contain too much organic character for use in some applications. Specifically, silver laurylsulphate has been observed to be extremely discolored when added into even a relatively low temperature (about 170° C.) melt of a polyolefin. This result is typical of polymer melt additives that either contain too much unstable organic character or are simply added in an excessive amount.
An antimicrobial masterbatch formulation is disclosed in JP 2841115B2 wherein a silver salt and an organic antifungal agent are combined in a low melting wax to form a masterbatch with improved dispersibility and handling safety. More specifically, silver sulfate was sieved through a 100 mesh screen (particles sizes less than about 149 microns), combined with 2-(4-thiazolyl)benzimidazole and kneaded into polyethylene wax. This masterbatch material was then compounded into polypropylene, which was subsequently injection molded into thin test blocks. These test blocks were reported to be acceptable for coloration and thermal stability, while exhibiting antibacterial properties with respect to E. coli and Staphylococcus, and antifungal properties with respect to Aspergillus niger. Similar masterbatches are also described in JP 03271208, wherein a resin discoloration-preventing agent (e.g. UV light absorbent, UV light stabilizer, antioxidant) is also incorporated.
An antimicrobial mixture of zinc oxide and silver sulfate on an inorganic powder support is disclosed in JP 08133918, wherein the inorganic powder support is selected from calcium phosphate, silica gel, barium sulfate and titanium oxide. The average primary particle diameter of the inorganic carrier powder is preferably ≦10 microns, more preferably ≦5 microns. The amount of silver sulfate supported is ≧0.04% and <5.0%, especially preferably ≧0.3% and <1.8%. The ratio of zinc oxide to the inorganic powder supporting silver sulfate is selected from the range of 0.2:99.8 to 30:70. The low overall content of antimicrobially active silver sulfate (<5.0%) in these particulate mixtures requires a relatively high loading of the mixture into a polymer or other substrate.
Silver sulfate has been proposed as an antimicrobial agent in a number of medical applications. Incorporation of inorganic silver compounds in bone cement to reduce the risk of post-operative infection following the insertion of endoprosthetic orthopaedic implants was proposed and studied by J. A. Spadaro et al (Clinical Orthopaedics and Related Research, 143, 266-270, 1979). Silver chloride, silver oxide, silver sulphate and silver phosphate were incorporated in polymethylmethacrylate bone cement at 0.5% concentration and shown to significantly inhibit the bacterial growth of Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa. Antimicrobial wound dressings are disclosed in U.S. Pat. No. 4,728,323; wherein a substrate is vapor or sputter coated with an antimicrobially effective film of a silver salt, preferably silver chloride or silver sulfate. Antimicrobial wound dressings are disclosed in WO2006113052A2; wherein aqueous silver sulfate solutions are dried onto a substrate under controlled conditions to an initial color, which is color stable for preferably one week under ambient light and humidity conditions. An antimicrobial fitting for a catheter is disclosed in U.S. Pat. No. 5,049,140; wherein a proposal to fabricate a tubular member composed of a silicone/polyurethane elastomer in which is uniformly dispersed about 1 to 15% wt. of an antimicrobial agent, preferably silver sulfate, is described. A moldable plastic composite comprising cellulose and a urea/formaldehyde resin is disclosed in WO2005080488A1, wherein a silver salt, specifically silver sulfate, is incorporated to provide a surface having antiviral activity against SARS (severe acute respiratory syndrome) coronavirus.
Despite various references to the proposed use of silver salts as antimicrobial agents in various fields as referenced above, there are limited descriptions with respect to approaches in the prior art for preparing silver salts, specifically silver sulfate, of sufficiently small grain-size and of optimal grain-size distribution as may be desired for particular applications. A need exists, in particular, to provide antimicrobial agents such as silver salts, more specifically silver sulfate, in controlled particular sizes for use in plastics and polymer containing materials with improved antimicrobial efficacy and reduced cost, while avoiding discoloration and enabling more robust manufacturing processes. A process of preparing silver sulfate with improved thermal stability that is simple, cost effective, and contains less organic material as an additive is desired.